This invention relates generally to energy conversion systems for generating electricity and to methods of controlling such systems, and more particularly to a variable speed generation (VSG) system and control method utilizing a brushless doubly-fed generator which operates in both an induction mode and a synchronous mode. Such a brushless doubly-fed machine has a winding structure different than four other known types of electrical machines.
First, a synchronous machine typically has one set of polyphase power windings on a stator which are connected to a grid. The synchronous machine has DC (direct current) field or excitation rotor windings terminating at slip rings on the rotor. Via the slip rings, the excitation windings are connected by brushes to an external DC excitation source. Such brushes require periodic maintenance and replacement, which typically requires that the synchronous machine be disconnected from the power grid and the rotor brought to essentially a standstill. The brushes and slip rings also increase the electrical losses of the machine which detracts from the overall machine efficiency.
A synchronous machine is restricted to operation at a particular synchronous speed, and if torque requirements are exceeded the machine will lose synchronism, commonly referred to as "slipping a pole." Thus, a synchronous generator is not well suited to variations in the resource energy received from an inherently variable resource, such as hydro or wind.
Secondly, an induction machine also has polyphase power windings on a stator which are connected to a grid. The induction rotor may be either a wound rotor having rotor windings or a squirrel cage rotor having conducting bars embedded in slots on the rotor and interconnected at each end by conducting rings. An induced polyphase current circulates through the wound rotor windings or the squirrel cage rotor bars.
An induction generator suffers from several major limitations, including an inflexible nature concerning speed variation. Speed control by varying the rotor resistance is inefficient. Speed control by varying the stator voltage and/or the stator frequency requires a power electronic converter having the same rating as the generator, placed between the stator terminals and the electric power grid. Thus, such a power electronic converter carries the full load current, which increases the operating costs due to power losses within the converter, as well as the initial cost of purchasing the system. Moreover, an induction generator requires a reactive power input. Thus, additional equipment is required, such as a static VAR system, which further increases the initial and operating costs. As a further disadvantage, the induction generator is inherently incapable of generating a reactive power output.
Thirdly, a hybrid electrical machine is a doubly-fed generator with brushes which has a stator with polyphase power stator windings connected to a grid. The hybrid doubly-fed machine has a wound rotor with polyphase excitation rotor windings terminating at slip rings. Via the slip rings, the rotor windings are connected by brushes to an external energy converter. This varies from the synchronous and induction machines discussed above, in that the energy converter varies the amplitude and frequency of AC rotor current or voltage to control the rotor speed and the output characteristics of doubly-fed machine. A controller monitors various system inputs and outputs, and according to a desired control strategy provides amplitude and frequency control signals to the energy converter. Such a doubly-fed generator is described in a pending United States application, of which the inventor of the present application is one of the coinventors, entitled "Doubly-fed Generator Variable Speed Generation Control System", Ser. No. 07/304,044.
However, this hybrid doubly-fed machine requires brushes to transfer the excitation power from the energy converter to the polyphase rotor windings. The same disadvantages of brushes used on synchronous machines are also present in the hybrid doubly-fed machine, namely: maintenance and replacement costs including parts, labor and machine down-time; and decreased efficiency from the additional electrical loses imposed by the brushes and the brush/slip ring interface.
Finally, a cascade induction motor for low speed operation was developed in the late 1800's. The original cascade machine was a two-frame machine comprising two wound rotor induction motors mechanically coupled together and coupled to the load. The primary motor slip power, that is the power extracted from the rotor windings of the primary machine, excites either the secondary machine stator windings or the rotor windings via slip rings and brushes. When the rotor of the secondary machine is excited, external resistances are added to the stator windings to control the speed. A rectifier-invertor returns the excess slip frequency power to the grid. By adjusting the invertor firing angle, the effective voltage and slip as seen from the secondary machine stator terminals is varied.
Brushless machines having two sets of polyphase stator systems with separate or shared common stator windings, and a squirrel cage rotor, have been studied and proposed by others in the past, particularly in the motor context. However, the extent of these studies has been to the use of the machine in a singly fed mode, that is, an induction mode. Any attempts at synchronous operation were limited to a single synchronous speed.
These prior studies and papers, discussed further below, are vague on the design aspects of the brushless machine, especially concerning the pole pitch of the windings. Generally, these machines had one set of stator windings connected to a power grid, and the other set of stator windings connected to a bank of variable resistors which were used to control the machine speed. The electrical losses produced by these resistors resulted in a very inefficient machine. Additionally, the use of the brushless machine in a singly fed or induction mode resulted in a poor power factor.
Power factor refers to the relative phases in the time domain of the waveforms of the polyphase AC voltage and current. For example, unity power factor is achieved when the waveforms of the polyphase AC voltage and current are in phase, that is, neither waveform is leading or lagging the other waveform. Operation at a poor power factor, such as a lagging or inductive power factor wherein the current waveform is lagging in time behind the voltage waveform, results in greater electrical losses within the machine, as well as on the lines feeding the machine, which detract from the overall machine efficiency.
The first discussion of which the inventor is aware of a brushless machine having two sets of three phase stator systems with common windings appears in a 1907 article by Hunt. (J. L. Hunt, "A New Type of Induction Motor," Proceedings of the IEE, Vol. 39, pp. 648-667, 1907.) A new induction motor was proposed by Hunt to overcome the limitations of prior single frame cascade induction motor designs.
The motor described by Hunt has a stator with a single stator winding having terminals for connection to a grid. The stator windings are grouped in parallel circuits and have taps, but are otherwise of the ordinary type. The parallel circuits are shared by each of the two three phase systems. The taps are connected in pairs through resistances during starting or during rheostatic speed control, and the taps are short-circuited at normal speed. This varies from the start-up control of the torque and speed of a conventional slip ring motor wherein the resistances are connected to the rotor windings by the slip rings, and not to the stator windings.
The rotor of the Hunt motor has a short-circuited winding comprising shorted rotor bars. This provides a rotor which is mechanically simple and approximates the construction of a squirrel cage rotor which is commonly used in induction motors. If the rotor has no slip rings, that is, a brushless rotor, the motor runs at one efficient speed. The Hunt motor may operate at two, three or four efficient speeds by using slip rings and varying the number of rotor poles with an external switching means. Thus, the Hunt motor does not operate at any desired speed, but only at a few fixed speeds.
The Hunt article also discloses that if continuous or DC current is supplied to one set of stator windings and AC current to the other set, the motor will run at one single synchronous speed like a conventional synchronous motor. In this case, the machine is started from a standstill as an induction motor until normal speed is reached. At this point, the DC current is applied to one set of stator windings by closing a switch, and the motor pulls into step for operation at synchronous speed. The Hunt motor does not operate at variable speeds while operating in a synchronous mode.
In a 1921 article by Creedy, additional developments in multi-speed cascade induction motors were examined, including faster motors, and those having a greater number of discrete stepped speeds with smaller intervals between them. (F. Creedy, "Some Developments in Multi-Speed Cascade Induction Motors," Proceedings of the IEE, Vol. 59, pp. 511-532, 1921.) In a conventional manner, intermediate speeds between the discrete steps are obtained by adding resistance to the rotor windings across the slip rings. Such resistances, of course, detract from the overall machine efficiency by nonproductively consuming power and dissipating it as heat (I.sup.2 R losses).
Basically, Creedy proposed a design method which removed some of the limitations of the Hunt design method. Creedy proposed two systems of stator windings, one having two poles and the other having six poles. In the discussion after the article, Hunt approves of the Creedy method and proposes using the two plus six pole stator winding configuration of Creedy with a brushless ("without slip-rings") rotor to obtain a commercial motor which runs at 750 rpm.
In a 1970 paper by Broadway and Burbridge, the rotor design of the Hunt induction motor was improved upon by producing a more efficient, robust, and economical squirrel-cage rotor. (A.R.W. Broadway and L. Burbridge, "Self-Cascaded Machine: A Low-Speed Motor or High-Frequency Brushless Alternator," Proceedings of the IEE, Vol. 117, No. 7, pp. 1277-1290, July, 1970.) The Broadway/Burbridge machine is a self-cascaded single-frame unit which operates in a manner equivalent to a two-machine arrangement, where one machine would have two p-poles, and the other machine, two q-poles. The fields of each machine share a common magnetic circuit and the currents, which are normally separate, share the same windings. Each phase of the stator winding carries two component currents, each generally of a different frequency and flowing in different paths within the same winding. However, each phase winding in the rotor carries a single current of only one frequency.
The Broadway/Burbridge machine may run asynchronously using resistance control, or synchronously without external rotor winding connections; that is, without slip rings or rotating diodes. For synchronous operation, the two q-pole components of the single-stator winding are energized with direct current. Broadway and Burbridge refer to this as being "doubly-fed," where a single stator winding receives both alternating (AC) current and direct (DC) current. The authors emphasize that "it is clearly desirable that no alternating current should flow in the direct-current supply, and vice versa." Thus, Broadway and Burbridge's use of the term, "doubly-fed" is quite different from that used herein to describe a preferred embodiment of the subject invention.
The Broadway/Burbridge rotor is a multicircuit single-layer winding with each slot containing only one conductor. A single rotor winding may be formed by a U-shaped conductor occupying two slots, with the ends of the U-shaped conductor shortcircuited by a squirrel-cage end ring. Several such U-shaped windings of increasing size may be grouped concentrically in adjacent rotor slots. For example, the innermost U-shaped winding may have one slot between the legs of the U-shaped coil, and this central slot may be left unoccupied. Thus, the Broadway/Burbridge rotor is simple to construct and highly durable or robust.
To avoid an unbalanced magnetic pull which occurs when two fields differ by only two poles, Broadway and Burbridge state that it is "essential" that the two main fields differ by a minimum of four poles. The highest speed operation of this machine as a motor, and the lowest frequency output as an alternator, exist when the two fields are of six poles and two poles. For example, four such groups of concentric U-shaped windings may be used for a (6+2)-pole rotor winding.
In a 1978 article, Kusko and Somuah proposed speed control of a self-cascade single-frame brushless induction motor using a frequency converter comprising a DC-linked rectifier-invertor. (A. Kusko and C. B. Somuah, "Speed Control of a Single-Frame Cascade Induction Motor with Slip-Power Pump Back," IEEE Transactions on Industrial Applications, Vol. IA-14, No. 2, pp. 97-105, 1978.) The frequency converter provides slip recovery or slip-power pump back; that is, excess power not required at a particular speed is fed back into the grid which supplies power to the motor. This motor comprises two wound-rotor induction motors which are built on a common set of core laminations within a single motor frame. The rotor windings of these two motors are interconnected on the single rotor to form a self-cascade system. The rotor has no slip rings or brushes and each rotor winding is described as consisting of one squirrel-cage bar and two short-circuited single conductor coils.
The stator has two sets of stator windings. The primary stator winding has the greater number of poles, for example four, and is connected to the grid. The secondary stator winding has the lesser number of poles, such as two, and is connected to the solid-state rectifier-invertor. The synchronous speed of the motor is set by the sum of the pole pairs of the primary and secondary stator windings. Thus, synchronous operation occurs at a single synchronous speed.
The Kusko/Somuah motor achieves a speed variation by removing the excess power from the stator through the secondary windings and applying it to the rectifier-invertor of the frequency converter. The power as received by the rectifier- invertor is AC and appears in a variety of frequencies. The rectifier portion of the frequency converter rectifies this wild frequency AC power into DC power which travels through a DC link to the invertor. The invertor portion then converts the DC power into AC power at the grid frequency for supply back into the power grid. Thus, there is no real control of the motor, just a recovery of the surplus power not required by the motor. According to the authors, the maximum benefit of the slip-energy recovery system is only attained at low speeds.
Kusko/Somuah state that operation of their device as a generator is impossible due to the unidirectional properties of the bridge rectifier diodes. However, if this limitation were overcome, operation as a generator occurs above a certain maximum speed only if there is a power reversal from the invertor to the rectifier in the DC link. That is, output power flows out of the primary (four-pole) stator winding and into the grid. This output power is equal to the total slip-frequency power flow into the secondary (two-pole) stator winding plus the mechanical power received from the turbine less the electrical and mechanical losses.
The Kusko/Somuah motor suffers from several limitations which would apparently limit a generator of the Kusko/Somuah design. For example, the frequency converter must be sized to handle the slip power which is proportional to the maximum slip times the load torque. Such a large frequency converter increases the cost of the overall system. Only the wide-speed range drives are capable of being started with the frequency converter. However, these wide-speed range drives suffer from a reduced power factor at part speed.
The Kusko/Somuah motor is unsuitable for driving varying mechanical loads. From an examination of the equations given by the authors, it is apparent that if the mechanical load driven by the motor varies, the speed of the motor will also vary. That is, such load variations will cause a shift in the torque-speed characteristic of the motor. Correction of this torque-speed characteristic requires injecting a voltage. That is, the voltage is injected at the second set of stator three phase systems. Therefore, control of the Kusko/Somuah motor also requires a speed control feedback loop, which monitors the shaft speed and supplies firing control to the frequency converter. Finally, the slip power control method not only requires reactive power for the converter, but also renders it impossible for the generator to generate reactive power. This is essentially because this method operates the machine in a singly-fed mode, i.e. the machine behaves like a conventional induction machine.
Thus, a need exists for a new VSG system, controller and method of controlling such a system to exploit the attractive features of a brushless doubly-fed generator, and to provide electrical power at a maximum efficiency from an energy source, such as the variable alternative energy resources of hydro and wind.